Aqueous dispersions of polyurethane of polyurethane ureas are known (e.g., as described in e.g. Angewandte Chemie, 82, (1970) pages 53 to 63 and U.S. Pat. Nos. 3,920,598 and 3,905,929). These dispersions are of a high quality.
An important reason for this high quality level is the fact that many of these dispersions are free from emulsifiers. They contain chemically incorporated hydrophilic centers which make the otherwise hydrophobic elastomers self-emulsifiable. This method of making dispersions self-emulsifiable has two major advantages over the use of emulsifiers:
(1) a smaller quantity of hydrophilic centers is required; and PA1 (2) the built-in emulsifier is incapable of migrating from shaped products produced from these elastomer dispersions; such migration normally has a considerable effect on the properties of a product. PA1 R represents a divalent group of the type which can be obtained by removal of the isocyanate groups from a diisocyanate of the formula R(NCO).sub.2 as defined above, PA1 R' represents hydrogen or a monovalent hydrocarbon group having from 1 to 8 carbon atoms, preferably hydrogen or a methyl group, PA1 R" represents a monovalent hydrocarbon group having from 1 to 12 carbon atoms, preferably an unsubstituted alkyl group having from 1 to 4 carbon atoms, PA1 X represents the divalent radical obtained by removal of the terminal oxygen atom from a polyalkylene oxide chain having from 5 to 90, preferably 20 to 70 chain members, of which at least 40%, preferably at least 65% consist of ethylene oxide units and which in addition to ethylene oxide units may also contain other alkylene oxide units, such as, propylene oxide, butylene oxide or styrene oxide units, propylene oxide units being preferred among the latter, PA1 Y represents oxygen or --NR''', in which R''' has the same definition as R", and PA1 Z represents a group having the same definition as Y. PA1 R, X, Y and R" have the meanings already specified. PA1 U represents O--CO--NH, NH--CO--NH, NH--CO or S--CO--NH and R, X, Y, R" have the meaning indicated above. PA1 (1) Salts of strong inorganic and organic acids with inorganic or organic bases; PA1 (2) salts of strong inorganic and organic bases with inorganic or organic acids; PA1 (3) strong inorganic and organic acids; PA1 (4) strong inorganic and organic bases.
The first feature, in particular, considerably reduces the sensitivity to water of shaped products produced from self-emulsified polyurethanes. The hydrophilic centers incorporated into the known polyurethanes or polyurethane ureas which are dispersible in water may be either salt-type groups, i.e. ionic groups, or non-ionic groups.
The first of these two types of polyurethanes, known as, "polyurethane ionomers", include both polyurethanes which have chemically fixed cations, i.e. in particular chemically incorporated ammonium ions, and polyurethanes which have chemically fixed anions, i.e. in particular chemically incorporated sulfonate or carboxylate groups. Among the last mentioned non-ionic polyurethanes which are dispersible in water may be included, in particular, the polyurethanes and polyurethane ureas described in U.S. Pat. Nos. 3,920,598 and 3,905,929 which contain polyethylene oxide side chains.
The dispersions of these polyurethanes have various characteristic properties depending on the nature of the hydrophilic center. Polyurethane ionomer dispersions are stable to elevated temperatures up to their boiling point because the solubility of the salt groups contained in them is virtually independent of the temperature. Non-ionic dispersions coagulate even when heated to moderate temperatures (about 60.degree. C.) because the polyethylene oxide side chains gradually lose their solubility in water at elevated temperatures. However, in contrast to ionomers, these dispersions are stable to the addition of virtually unlimited quantities of electrolytes as well as to freezing and thawing.
Polyurethane dispersions and polyurethane solutions on the basis of ionomers containing significant quantities of water-soluble electrolytes, e.g. more than 0.4 g per 100 g of polyurethane, have not hitherto been known. Indeed, the presence of electrolytes has hiterto been deliberately excluded as far as possible in order not to reduce the stability of the dispersions and solutions. In most ionomer disperisons known in the art, even the addition of a small quantity of aqueous electrolyte solution immediately causes localized coagulation. If such coagulation does not occur instantly, for example if it occurs only after several hours or days, as is the case with dispersions containing sulfonate groups, the dispersions are already considered to have good stability to electrolytes.
The present invention provides new, stable aqueous dispersions or solutions of polyurethane ionomers which contain water-soluble inorganic or organic electrolytes.